A heat-developable photosensitive material comprises a support and a composition applied thereto which is obtained by dispersing mainly a fatty acid silver salt and an organic reducing agent, optionally together with a small amount of a photosensitive silver halide, in a polymer binder.
The silver halides so far used widely are excellent in photographic characteristics, hence are utilized in high quality image forming materials. However, they have problems; the procedures in the steps of development and fixation are complicated and, since the above steps each involves wet treatment, not only troubles are caused by the complicated treatment works but also chemical waste liquid is discharged in large amounts.
For avoiding such problems, heat-developable photosensitive materials have been developed so that the development step may be carried out not in the manner of wet process but in the manner of heat-development, and have partly been put to practical use.
Thus, for example, Japanese Kokoku Publication Sho-43-004924 discloses a heat-developable photosensitive sheet material which comprises an organic silver salt containing, within the molecule, a group of associated silver ions, a silver halide catalytically contacting with the silver ions, and a slow reducing agent.
In the above-cited publication, it is describe that the heat-developable photosensitive sheet material is formed by incorporating a photosensitive material comprising such organic silver salt, silver halide and slow reducing agent in a film or a fibrous product almost free of any binder, which serves as a transparent self-support, or by incorporating the above material in a coat layer of a film-forming binder, such as polyvinyl butyral, polymethyl methacrylate, cellulose acetate, polyvinyl acetate, cellulose acetate propionate or cellulose butyrate, and applying the resulting composition to a heat-resistance support, such as a paper sheet, plastic film, metal foil or glass sheet; and polyvinyl acetal resins are used and alleged to be most suitable as the above film-forming binder.
However, for reasons of production process, the conventional polyvinyl acetal resins contain minute amounts of impurities, and these impurities cause the film-forming binder itself to have photosensitivity, so that the following defects may be caused: the coat layer of the film-forming binder prepared may be colored unreasonably or, after application, images may undergo or show fog, poor gradation or deficient sensitivity, or the photosensitive sheet material may have poor storability before image forming. Depending on the glass transition temperature of the polyvinyl acetal resin, the film may undergo heat deformation in the step of heat-development, or images after application may show fog, poor gradation or deficient sensitivity, or cracking may occur during film handling, among others.
As means for solving the above problems of deterioration of image characteristics, the use of a thione compound represented by a general formula as the silver salt oxidizing agent in a thermal photographic composition containing a photosensitive silver halide, a reducing agent and a silver salt oxidizing agent is described in Japanese Kokai Publication Sho-49-052626, for instance, by which stable images can be formed without causing any independent stabilizer or stabilizer precursor to exist in the system. However, the use of such thione compound as the silver salt oxidizing agent can never solve the above-mentioned problems which the conventional polyvinyl butyral resins have.
Further, for improving image characteristics, it is said to be necessary that the fatty acid silver salt, organic reducing agent, photosensitive silver halide and other ingredients be dispersed more uniformly. However, when a polyvinyl acetal resin with a low degree of polymerization is used for making a point of the dispersibility, the resulting coat will be weak in strength and problems such as fog may arise. When, conversely, a polyvinyl acetal resin with a high degree of polymerization is used for satisfying the coat strength requirement, it becomes difficult to uniformly disperse the fatty acid silver salt, organic reducing agent and photosensitive silver halide therein.
Furthermore, among heat-developable photosensitive materials, heat-developable silver salt films are inferior in image characteristics, in particular image density and visibility of gradated portions, to the conventional X-ray sensitive films in which gelatin is used and which is processed by the wet method. Therefore, it is strongly demanded that the heat-developable silver salt films be improved with respect to these image characteristics. For that purpose, it is necessary to strictly control the nuclear growth of silver in the step of heating.